Our evaluation of patients undergoing PCI in a national, random sample of hospitals between 1998 and 2000 demonstrates that patients treated at medium-volume hospitals (200 to 399 cases annually) and patients treated at hospitals with 400 or more PCI cases had statistically comparable mortality rates. These findings challenge earlier reports of poorer outcomes for patients undergoing PCI at middle-volume centers (7
). Our results further suggest that efforts by the Leapfrog Group advocating a minimum annual hospital PCI volume of 400 cases are unlikely to achieve sizable reductions in PCI-related patient mortality (4
). Moreover, our findings do not support current ACC/AHA PCI clinical practice guidelines recommending hospitals maintain a minimum annual PCI volume of 400 cases (3
Our findings should not be interpreted as evidence that there is no association between hospital PCI volume and mortality. Indeed, we found that patients treated at hospitals that performed fewer than 200 PCI procedures annually had a higher risk of mortality than patients at hospitals in any other PCI volume group. At the same time, we noted substantial heterogeneity in hospital SMRs within each of the hospital PCI volume groups, and failed to find a statistically significant difference in mortality risks among patients treated in medium, high, or very high PCI volume hospitals. Given that we evaluated over 360,000 admissions at 457 hospitals throughout the U.S. over a three-year period, we believe this analysis, unlike previous reports of a neutral volume-outcome association (22
), was sufficiently powered to detect even a modest difference between hospital PCI volume groups (3
). If our analysis failed to detect a volume-mortality association, the magnitude of any such effect would have been so small as to be virtually meaningless from either a clinical or policy perspective. Finally, the similarity in mortality rates between medium, high, and very high hospitals across a variety of clinical strata suggests these results are robust and not limited to selected patient groups.
There are several potential explanations for the comparable mortality rates among patients treated in medium, high, and very high PCI volume hospitals. Percutaneous coronary intervention technology has changed markedly since the first studies of hospital PCI volume and outcomes (14
). Although the adoption of coronary stents during the mid-1990s improved patient outcomes (13
), previous studies indicated that patients undergoing PCIs involving stents at higher volume centers continued to have lower mortality rates than patients undergoing PCIs involving stents at lower volume centers (13
). However, these studies evaluated procedure use before 1998. Stents may have attenuated the hospital PCI volume-mortality difference between medium-, high-, and very high-volume hospitals in subsequent years as additional operators were trained in their use, operators became more experienced, and newer stent designs were incorporated into practice. In addition, the increased use of new adjunctive pharmacologic therapies in the late 1990s, including glycoprotein IIb/IIIa receptor inhibitors (34
) may have reduced the mortality difference between medium, high, and very high PCI volume hospitals. As in the case of stents, the full beneficial effect of these therapies may not have been observed in previous studies (13
Technological improvements in PCI have been accompanied by significant changes in PCI practice patterns in the past decade (14
). The substantial increase in the number of PCIs performed reflects a rise not only in the number of providers performing PCIs (i.e., diffusion), but also in the total number of procedures ever performed by each provider (i.e., accumulated experience) (26
). This cumulative gain in provider experience may explain why mortality rates are now similar across medium, high, and very high PCI volume hospital groups. Additionally, the clinical threshold for PCI use has changed over time (14
). To the extent that increases in PCI volume have occurred as a result of the treatment of a lower risk patient pool, any mortality difference between hospital PCI volume groups would have become smaller as the overall absolute mortality rate decreased. Alternatively, data from California indicate that some smaller PCI volume hospitals stopped providing PCIs during the 1990s (24
). The lack of a difference in this study in mortality rates between medium, high, and very high PCI volume hospitals may be because medium-volume hospitals that stopped providing PCI were, on average, hospitals with higher mortality rates. Finally, differences in mortality between hospital PCI volume groups may be both smaller and less well established than commonly acknowledged. A recent literature review noted that nearly half of the published studies evaluating hospital PCI volume and outcomes had reported no association with in-hospital mortality, while the remainder suggested only a modest mortality difference (35
We believe our findings have important implications concerning the use of hospital volume as a proxy for PCI quality. Our findings, along with evidence of the benefits of hospital competition (36
), the economic consequences of restricting PCI provision (37
), and patients’ documented willingness to tolerate increased mortality rates at local hospitals rather than travel to lower mortality rate regional hospitals for treatment (38
) challenge the benefit of a hospital PCI volume standard of 400 annual cases (4
). Further, the current PCI volume minimum criterion would unfairly affect the majority of hospitals in the medium-volume group with outcomes that are better than or comparable to what would be expected given their patient case-mix (i.e., SMR ≤1).
Our findings also underscore the importance of periodic reevaluation of the volume-outcome association in order to allow for changes in technology and practice patterns. Recent data supporting the increased use of interventional strategies in the management of unstable angina (39
), benefits provided by new pharmacologic therapies (40
), and the promise of drug-eluting stents (41
) make it apparent that PCI practice is continuously evolving. This is consistent with the pattern of decreasing differences in mortality between hospital PCI volume groups (24
). Thus, the suggestion that there is sufficient evidence to “move ahead” with existing volume standards (42
) may, in the case of PCI, be premature given the dated information upon which those standards are based. Future studies of the PCI volume-outcome association will be needed to identify and test new volume thresholds, and ultimately to determine the process(es) through which volume and outcomes are linked.
There are four potential limitations of our evaluation that merit specific consideration. First, our analysis used administrative data, and thus may not have captured the full clinical detail of a patient’s risk profile. However, it is unclear whether having additional clinical data would have led us to uncover a meaningful difference in mortality between medium- and high-volume PCI hospitals. Given that patients treated at low- and medium-volume hospitals had a higher risk profile based on this administrative data, it is likely that more detailed clinical risk adjustment would have further narrowed any putative hospital PCI volume-associated mortality difference.
Second, we evaluated in-hospital mortality alone and could not assess other patient outcomes, including periprocedural complications, repeat revascularization rates, or longer-term outcomes. However, current ACC/AHA PCI clinical practice guidelines recommend hospital PCI volume minimums primarily on the basis of prior studies demonstrating a reduction in procedure-related mortality (3
). Similarly, the Leapfrog Group explicitly justifies hospital PCI volume minimums as a means of reducing mortality (4
). In addition, long-term outcomes are reportedly comparable for patients treated at different PCI volume hospitals, suggesting that any differences in outcomes between PCI volume hospitals are observed during hospitalization (22
Third, the NIS does not track the experience of individual physicians. Thus, we cannot account for the influence of individual operator PCI volume on the association between hospital PCI volume and mortality. Physician PCI volume may modify the association between hospital PCI volume and outcomes, particularly if physicians at low-volume hospitals practice at more than one hospital. However, prior studies suggest both volume measures influence patient outcomes independently (9
), and current ACC/AHA guidelines recommend separate operator and hospital annual PCI volume minimums (3
). More importantly, if individual physician PCI volumes account primarily for the association between hospital PCI volume and outcomes, then efforts to reduce PCI mortality should focus on operator, and not hospital, volume. In addition, patients undergoing PCI who did not require hospitalization would not be represented in our sample, although it is uncertain what proportion of patients are in this group and how they are distributed across hospital PCI volume groups.
Finally, the NIS database does not contain unique patient identifiers. The inclusion of multiple patient admissions in our cohort may violate the statistical assumption of independence of observations. However, patient readmissions would alter our findings substantively only if readmissions were not randomly distributed across hospital PCI volume groups, and if readmission constituted an independent mortality risk beyond that measured by our current covariates.
Our evaluation of more than 360,000 PCIs from a national random sample of U.S. hospitals between 1998 and 2000 found no difference in mortality rates for patients treated at medium (200 to 399 cases/year), high (400 to 999 cases/year), and very high (≥1,000 cases/year) PCI volume hospitals after accounting for differences in case-mix. Moreover, within each of the hospital PCI volume groups, we detected substantial heterogeneity in hospital mortality. These results contradict current ACC/AHA clinical practice guidelines recommending all hospitals perform 400 PCIs each year. Our findings also question the usefulness of efforts by the Leapfrog Group to adopt PCI hospital volume minimums as a means of improving quality of care. Future evaluations of hospital PCI volume thresholds will be needed to assess recent and pending changes in PCI technology and practice.